Aircraft flight control surfaces (e.g., flaps, rudder, ailerons, elevators, etc.) are typically controlled by electrohydraulic servoactuators operating in closed-loop position servosystems. Such systems determine the error between a commanded position and an actual position, and operate the servoactuator so as to reduce or eliminate the error.
The conventional two-stage servovalve typically has an electrical section and a hydraulic section. The electrical section includes a torque motor which is arranged to produce a proportional pivotal displacement of an armature-flapper member in response to an input current. The armature-flapper member is commonly mounted on a substantially frictionless flexure tube. Hence, an input current of one polarity will displace the flapper in one angular direction, while a current of the opposite polarity will displace the flapper in the opposite direction. In the hydraulic section, the flapper portion is movable between two spaced, facing nozzles. Thus, as the torque motor causes the flapper to move closer to one nozzle and farther from the other, back pressures are developed in the fluid conduits leading to these nozzles. The differential of these back pressures is, in turn, used to selectively displace a second-stage valve spool relative to a body to create the desired flow(s) through the valve. The valve may also include a mechanical feedback wire which functions to urge the flapper back toward a centered or null position between the nozzles when the spool is in the commanded position. Such servovalves are commonly used to control the flows of fluid with respect to the opposing chambers of a double-acting fluid-powered actuator. Additional details as to the structure and operation of such a servovalve are more fully shown and described in U.S. Pat. No. 3,023,782, the aggregate disclosure of which is hereby incorporated by reference.
In aircraft, the servovalve is typically located immediately adjacent the actuator. This has required long lengths of electrical conductors from the source of the command signal (typically, the cockpit) to the servovalve site. Moreover, it is common to use redundant control systems such that, should there be a failure of one conductor, effective control of the servovalve may still be maintained through the other undisturbed conductive paths. Indeed, many aircraft employ quad-redundant systems, in which four conductors follow physically separate paths from the command source to the servovalve. The length of such conductors, magnified by the extent to which redundancy is provided, has posed a problem in that such conductors are susceptible to electro-magnetic interference, such as lighting and the like. Any interference with the command signal can cause unintended servoactuator response.
To render such systems less susceptible to electro-magnetic interference, others have proposed to control the servovalve by an optical technique. This generally contemplates the transmission of an optical command signal, controlled by a pulse-width-modulated technique, along one or more optical fibers from the cockpit to the servovalve. While this has been conceptually feasible, it has not wholly solved the problem because there has still been a need to provide an electrical conductor to power the servovalve-driving amplifier or controller in response to such optically-transmitted command signal. For example, U.S. Pat. No. 4,422,180 appears to disclose a servosystem control system for aircraft applications, in which a command signal is transmitted optically to a remote servosystem. However, this system also contemplates the simultaneous transmission of electrical power via one or more conventional conductors to various parts and components of the system, such as a receiver, a demodulator, a processor, and servoamplifier. Thus, in this type of system, the command signal was transmitted optically, but electrical power was transmitted via conventional conductors.
Other details of known systems for transmitting optical command data between a source and a receiver, are shown in U.S. Pat. Nos. 4,313,226, 4,495,655, and 4,538,655.